With rapid development of semiconductor technology, integrated circuits having enhanced performances and functions require larger element density, and components such as CMOS devices and metal interconnects need to be further scaled down. Accordingly, the integration degree of devices needs to keep increasing. Isolation structure is a component for isolating active regions of respective elements. Currently, Shallow Trench Isolation (STI) is a typical isolation structure for isolating active regions. The STI is formed on one surface of a substrate and is a shallow trench in the substrate filled with dielectrics. Therefore, it is only possible to fabricate devices on one surface of the substrate, which is disadvantageous for increasing the utilization of substrate and the integration degree of devices.